Dual ring reconfiguration switching unit

ABSTRACT

A multiport access and reconfiguration unit for use in a dual token ring network is provided with three adapters and, depending on error conditions, can assume one of a plurality of recovery states in which a reconfigured single network is formed and at least one state in which an isolated single ring is formed in addition to the reconfigured ring.

This invention relates to serial communication networks and moreparticularly to a multistate reconfiguration and access unit for use ina dual ring serial network.

BACKGROUND

Serial communication networks provide many advantages over otherwell-known networks such as multipoint, star or mesh networks, the mostvaluable being a fair distributed polling function which readilysupports peer-to-peer communications amongst a large number of stationswith a high utilization of the network capacity.

One major drawback of the serial network is its propensity tocatastrophic failure when any component of the network fails.

Over the years many techniques have been developed to detect and/orisolate faults in serial communication networks. One of the more usefultechniques, which is in use today in the IEEE 802.5 Token Ring, isdisclosed in U.S. Pat. No. 3,564,145. This technique known as beaconingidentifies a station, by its address, immediately down stream of afailed network component or station. In a static network (e.g. one inwhich the network topology is fixed or known) corrective action can betaken to bypass or fix the failed network component.

Another technique (Dual Ring Reconfiguration) has proven very useful inthe isolation of faults in a serial network, thereby providing completeor partial network operation following the failure of a networkcomponent. This technique employs dual serial rings which may beconverted to single ring via switching means to thereby bypass a failednetwork component. The patents listed below disclose a variety of DualRing Reconfiguration implementations:

U.S. Pat. Nos. 3,519,750, 4,527,270 3,876,983, 4,538,264, 4,009,469,4,594,790, 4,354,267, 4,709,365, 4,390,984.

The patents listed below disclose a variety of manual and automatictechniques for bypassing a failed network component in a single ringserial network:

U.S. Pat. Nos. 3,458,661, 4,035,770, 4,048,446, 4,245,343, 4,763,329.

While all of the techniques described above are useful either bythemselves or in combination, they are unable to provide fast, efficientor complete restoration of communications in a serial ring networkfollowing failures of aH kinds.

Modern serial networks such as the IEEE 802.5 Token Ring Networkgenerally employ many (up to several hundred or more) ports locatedthroughout an establishment. Many of these ports are not utilized or areconnected to inactive stations. In addition, stations (each of whichincludes a unique identity or address) are frequently moved from oneport to another for the convenience of the operator.

In view of the mobility of the stations and the large number of portswhich have no or inactive stations connected, the station identity oraddress accompanying a beacon message provides little information tolocate the geographic position of the failed network component.

The technique (Next Active Upstream Neighbor) disclosed in U.S. Pat. No.4,507,777 is very useful in managing fault recovery in serial networks;however, the sequential station identities or addresses derived fromthis technique do not provide sufficient network topology information toaccurately pinpoint the physical location of the failed component. Forexample, two adjacent active stations may be separated on the physicalnetwork by a number of non-connected or inactive ports. Thus, knowingthat station X detected a failure and that station C preceded X does notphysically locate a particular faulty component.

Current serial ring communication networks have no automated provisionfor developing an accurate network topology which accurately associateseach physical port with the identity or address of an active stationconnected thereto. Two solutions, neither of which is practical, havebeen considered. Each station upon activation could input its identityor address along with a network defined location to a database ormanager. This task could be accomplished manually by an operator. Thisis a burdensome task and would require cooperative human operators atevery station. On the other hand, sufficient intelligence could beprovided at each port to automatically provide the location informationeither to the station for automatic transmittal to a database or networkmanager or directly when a station connected thereto becomes active.Such a solution is impractical from a cost standpoint alone.

SUMMARY OF THE INVENTION

The invention contemplates a multistate reconfiguration and access unitfor attachment to a dual ring serial network. The unit includes: inputsand outputs for connection to the primary and secondary rings of thedual network; three adapters for receiving and retransmitting and/orgenerating signals; a plurality of ports for attaching data terminals orthe like; a first controllable switching means for selectivelyconnecting the ports in series circuit; a second multistatereconfiguration switch connected to the inputs and outputs, the threeadapters and the first controllable switching means for interconnectingthe connected elements in one of a plurality of configurations inresponse to control signals; and a programmed processor connected to theadapters for receiving signals therefrom indicative of the operation ofthe network and for generating control signals corresponding thereto andfor applying the generated control signals to the second multistatereconfiguration switch for causing it to assume a corresponding one ofthe plural configurations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a dual ring network includingreconfiguration units contracted according to the invention;

FIG. 2 is a block diagram of the novel reconfiguration unit illustratingnetwork information signal flow;

FIG. 3 is a block diagram similar to FIG. 2 illustrating internalcommunication (non-network) within the reconfiguration unit;

FIGS. 4A-4D illustrate the different switching states of thereconfiguration unit; and,

FIGS. 5-17 are flowcharts illustrating the operation of the programmedmicroprocessor illustrated in FIGS. 2 and 3; and,

FIGS. 18-20 are flowcharts illustrating the operation of new functionsperformed in the adapters illustrated in the drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a dual token ring network which include fourreconfiguration units RCU1-RCU4. These units are interconnected by aprimary ring P and a secondary ring S. Each RCU is provided with eightyports and can attach up to eighty devices such as terminals T to theprimary ring during normal or no fault operation. When so connected, anydevice Ti can communicate with any other device Ti±a via the primaryring P provided the other device is connected and operational.

In the event of a failure of a component or link in the primary ring,the primary and secondary rings will be reconfigured via switchingnetworks in one or more RCU's to form a single ring over which all orsome of the devices T may continue to communicate until the failedcomponent is replaced or repaired. How this is accomplished will bedescribed hereinafter in conjunction with the description of thedrawings. In order to support reconfiguration, the unidirectional signalflows in the P and S rings are in opposite directions as indicated bythe direction arrows of the link segments P and S illustrated in FIG. 1.

FIGS. 2 and 3 are block diagrams of a single RCU. In FIG. 2 onlyinformation signal (or data) flow is illustrated while FIG. 3illustrates only internal communication between the various componentsof the RCU. The same reference numerals will be used for the samecomponents in both FIGS. 2 and 3.

A reconfiguration switch 11 is connected to the primary and secondaryrings, through adapters 10, 15 and 16 and four serially connectedsegment switches 12 and the eighty ports they serve. Switch 11 can be inanyone of the switching states illustrated in FIGS. 4A-4D and willinterconnect the adapters, the ports served by the segment switches. Amore detailed discussion of these states will be provided below;however, the description which follows will assume that switch 11 is inthe normal state illustrated in FIG. 4A.

The information signals passed on by PI adapter 10 are applied to afirst segment switch 12 via reconfiguration switch 11 over aconductor 1. Segment switch 12 is a relay operated multiple pole doublethrow switch which in one position connects conductor 1 to conductor 2and conductor 3 to conductor 4. In its alternate position, conductor 1is directly connected to conductor 4 and conductor 2 is connected toconductor 3.

Conductor 2 is connected to one input of a port module 13 which includestwenty ports to which a device such as terminal T can be connected. Eachport is provided with a relay operated multiple pole double throw switch14 similar to segment switch 12 and when in one state connects conductor1' to conductor 2' and conductor 3' to conductor 4'. Conductor 4' of thefirst port is connected to conductor 1' of the second port and thusforms a series connection of all ports having switches in this state.Conductor 4' of the last port switch is connected to conductor 3connected to the first segment switch 12.

If all of the ports are active (i.e. have active devices T connected)all of the switches 14 will be in the first state described and devicesT1-T20 will be connected in series and the information signals passed onby PI adapter 10 will pass serially through devices T1-T20 and returnedto the first segment switch 12. The second, third and fourth segmentswitches 12 and their associated ports and connected device will operatein a like manner.

Conductor 4 of the fourth segment switch 12 is connected to the input ofPO adapter 15 by reconfiguration switch 11 and the output of adapter 15is connected to the primary ring P(out).

In the normal mode, information signals arriving at P(in) pass in seriesthrough PI adapter 10, the first segment switch 12 and active connecteddevices T1-T20; thereafter in series through the remaining segmentswitches 12 and active connected devices T associated therewith and thenthrough PO adapter 15.

In the normal mode information signals on the secondary ring S passdirectly through S adapter 16 via reconfiguration switch 11 to theoutput side of the S ring. If all of the RCUs are in this mode, the Sring carries no information signals from the devices T. In the othermodes (FIGS. 4B-4D) information signals from the devices T traverseportions of both the primary and secondary rings which have beenreconfigured into a single ring.

Segment switches 12 and port switches 14 are controlled bymicroprocessor 17 via the solid line conductors 18 schematicallyillustrated in FIG. 3. Adapters 10, 15 and 16 communicate withmicroprocessor 17 over a bus 19 shown in dashed line in FIG. 3.

Adapters 10, 15 and 16 may be substantially similar in construction andoperation to the IBM 16/4 Token Ring Adapter/A. This adapter interfacesthe token ring media and attaches to the input/output bus of the IBM PCor compatible personal computer. Information exchanges between theadapter and the PC are accomplished by a shared memory technique.

In this technique a memory area within which each device may both readand write is provided. The devices periodically check the memory todetect changes which result when one of the devices alters part or allof the memory content. A portion of this memory area includes a numberof flag bits each of which is defined as to meaning and function.

In order to modify the adapter function, three (3) additional flags havebeen defined. A first flag (TBF) when set enables the adapter totransmit Beacon Reconfigure (BNR) frames. The BNR frames and the beaconnormal (BNN) frames are defined as to format and content by the IEEE802.5 standard. The second flag MBF instructs the adapter to transmitBNR frames in place of what it is receiving. A third flag DIS₋₋ TXinstructs the adapter to break its transmit path which is accomplishedby transmitting BNN frames in place of what it is receiving, ifanything. The functions performed by microprocessor 17 describedhereinafter and illustrated in the flow charts of FIGS. 5-17 may beexecuted by a properly programmed INTEL 80C186 microprocessor.

As previously described, the state of switch 11 illustrated in FIG. 4Ais the normal state of the RCU. That is no failures are detected. Inthis state of switch 11 all of the ports are connected in series circuitbetween adapters 10 and 15 on the P ring and adapter 16 is inserted intothe S ring.

If adapter 10 detects an upstream failure on the P ring, it generatesnormal beacon frames (BNN) and transmits BNN frames downstream via theseries circuit to adapter 15. In addition, it sends a message indicatingthis condition to microprocessor 17 via bus 19. A failure can takeseveral forms, the most common being no signal caused by a break in acable or a failure in a circuit component of an upstream RCU. At thistime, microprocessor 17 notes the failure and awaits additionalinformation before changing the state of switch 11.

Adapter 15 upon receiving a BNN frame, transmits a beacon reconfigureframe BNR downstream on the primary ring P(out). At the same time, itnotifies microprocessor 17 via bus 19. The message from adapter 15 isnoted and the microprocessor 17 sends a message via bus 19 to adapter 16which causes adapter 16 to transmit BNN frames downstream on the S ring.If both the upstream P and the downstream S rings are broken, the BNNframes from the adapter 16 will be undetected by any device. However, inthis case the next downstream adapter 16 on the S ring of the adjacentRCU will have detected a break or failure and have initiated thetransmission on the S ring of BNN frames. If on the other hand only theupstream P ring has failed, the BNN frames will reach the adapter 16 onthe next downstream RCU on the S ring.

The BNR frames from adapter 15 travel downstream and each adapter 10notifies its connected microprocessor 17 of the receipt of the BNRframe. The microprocessor 17 sets a timer and checks to see if itsassociated adapter 16 on the S ring has detected an upstream S ringfailure on received a BNN frame. If neither event has been reported byadapter 16, it will take no action until the timer expires.

If the time interval is long enough, taking into consideration thepropagation delay around the ring, the RCU immediately upstream of afailure in the primary ring P will have either detected a failure on thesecondary ring S or received a BNN on that ring and a BNR on the primaryring before any other intervening RCUs have timed out.

Upon satisfying the conditions set forth above, i.e. received BNR on Pring and either a BNN or detect failure on S ring, the microprocessorwill reconfigure switch 11 to the state illustrated in FIG. 4C. In thisstate, the received BNR frame will be looped back on the secondary ringS. As this BNR frame is received by intervening adapters 16 theassociated microprocessor resets the timeout period and remains in thenormal mode.

When adapter 16 of the RCU which detected the upstream P ring failurereceives the BNR frame initialized by its associated adapter 15, this issent to the microprocessor 17 which recognizes adapter 15's identity andwill after internal checks switch to the state illustrated in FIG. 4B.At this time the dual ring has been reconfigured and includes thedetecting RCU at one end (FIG. 4B); the RCU upstream of the break orfailure in the P ring at the other end (FIG. 4C); and as manyintervening RCU as are connected in the normal state (FIG. 4A).

The failed ring segment includes the adapter 10 which detected thefailure in the first place (FIG. 4B) and the adapter 16 of the RCUupstream of the failure on the P ring. These two adapters will send BNNframes which due to the failure will not be effective to restore dualring operation. Once the failure is removed, one adapter will receive acomplete BNN frame first and will retransmit this frame. When one ofthese adapters receives a BNN with its own identity, it knows theinoperative ring portion is complete or restored (i.e. the BNN framemade a round trip). At this time, normal recovery in accordance with theIEEE 802.5 standard is executed. When this is completed, the associatedmicroprocessors command switches 11 to the normal state (FIG. 4A) anddual ring operation is restored.

It should be noted that in FIGS. 4A, 4B and 4C all of the ports 1-80 inthe RCU are bracketed by two adapters. In FIGS. 4A and 4C adapters 10and 15 bracket ports 1-80 while adapters 16 and 15 bracket ports 1-80 inFIG. 4B. In the state illustrated in FIG. 4D the ports 1-80 areconnected to one adapter (16) only; however, they are isolated from allother ports on the network.

This circuit arrangement in conjunction with the ring poll function setforth in the IEEE 802.5 standard makes possible the correlation of thedevice identities by the microprocessor 17. The ring poll function isimplemented in IEEE 802.5 Token Rings and is briefly described below.When the ring is first started or has recovered from a fault (whichusually occurs when a device enters or leaves the ring), all of thedevices enter a process to select the active device on the ring havingthe highest address or identity as the active monitor while all of theother devices assume the roll of standby monitors.

Once the process is completed, the active monitor transmits an activemonitor present (AMP) frame. This frame includes a broadcast TO addressfield and its own address in the FROM address field. In addition, theframe includes a frame copied bit. Since the ring is serial, the nextactive downstream station receives the frame (since it includes abroadcast address) before any other station. Because the frame copiedbit is not set, it knows that the active monitor is its next activeupstream neighbor (NAUN) and stores this information. It also sets theframe copied bit before retransmitting this bit, thus subsequent deviceson the ring will not mistakenly assume that the active monitor is theirNAUN.

This device will wait for a token frame and will after receipt of atoken frame transmit a standby monitor present (SMP) frame. Like the AMPframe it includes its device address in the FROM address field and abroadcast address in the TO address field and a frame copied bit. Thenext active downstream device repeats the process described above.

The process repeats until the active monitor receives a SMP frame with aframe copied bit not set. At this time each active device on the ringhas acquired the address or identity of its NAUN and the ring pollprocess is complete; however, the existing process described above cannot identify or correlate the devices with the ports to which they areconnected since the information available to do this is inadequate. Anystation can copy the identities of the devices in order; however, itwould not obtain sufficient information to correlate the identities ofthe devices with any particular port or physical location.

According to the invention one adapter 10/15 passes all ring pollmessages received to microprocessor 17 including its own ring pollmessage. Thus in sequence microprocessor 17, as does the ring monitor,receives all of the identities of the devices on the ring; however,unlike the ring monitor it knows the bracketing identities of its ownadapters 10/15/16 and the status of the eighty ports there between andtherefore has sufficient information available to correlate theseidentities with the active ports. The correlation is deemed accurate ifthe number of identities or addresses received equals the number ofactive ports. If additional reliability is desired, the microprocessorcan wait for two ring poll cycles before declaring the correlationaccurate.

Once this information is obtained, it can be transmitted by themicroprocessor via one of the adapters (10/15/16) to a manager programsuch as the IBM LAN Network Manager program resident on one of thedevices. The manager program can in this manner receive from all theRCUs precise topology information for the entire network. One of theoutstanding advantages of the invention is the generation of thistopology information with no changes to existing devices or addition ofintelligent devices at each port.

In accordance with the IEEE 802.5 standard, a device connected to a portwhich wants to be connected to the network must provide a d.c. signal.This signal causes switch 14 to insert the device if the processor 17has enabled the port. The signal is also sent over bus 18 to theprocessor 17. Thus, the processor 17 has sufficient information tomaintain a table of active/inactive ports. The device will maintain thed.c. current as long as it is active and if it powers down or becomesinactive for any other reason, the d.c. is removed and microprocessor 17senses this condition and the loss of d.c. current switches 14 to itsalternate state or bypass and changes the status of the port to bypassedin the table.

During the ring poll process, microprocessor 17 builds a table whichincludes the identities of those stations whose identities are bracketedby the identities of the selected two of the three attached adapters(10, 15 and 16). In the absence of a failure (e.g. a device did notremove the d.c. insert signal but failed to respond in the ring pollsequence), the number of reporting stations will equal the number ofactive ports and since the identities are provided sequentially, theycan be readily correlated to the specific ports at which they reside.

Another source of apparent error can occur when a device goes activeafter the ring poll passes its port but before the ring poll sequencecompletes. In order to increase the reliability of the data collected,it may be desirable to allow two ring poll sequences to take placebefore considering the data to be accurate.

As can be seen from the above, the provision of the three adapters 10,15 and 16 permits implementation of the topology generation under theconfigurations illustrated in FIGS. 4A-4C and still provides for anadapter connected to the non-operative ring portion (FIGS. 4B and 4C)dedicated to recovery when the fault which occasioned the switch statechange disappears. In the state illustrated in FIG. 4D, the devicesconnected via adapter 16 are isolated from the remainder of the networkand all ring poll messages or any other information provided by adapter16 are all associated with devices connected to the ports whichmicroprocessor 17 controls.

In the isolated state (FIG. 4D) adapter 16 is connected in a single ringalong with any active devices connected to the RCU via its ports. Inaddition to being the power down state of the RCU, it will be enteredwhen a fault internal to the RCU and/or its connected devices or cablesis detected. This state allows the remainder of the network to operateas a single loop while the microprocessor 17 determines the location ofthe internal fault and restores local operation via operation of eitherone or more port switches 14 and/or operation of one or more segmentswitches 12 when appropriate.

A fifth state (similar to FIG. 4D except adapters S and PO areinterchanged) exists when two faults exist at the same time on the ring,one fault being upstream of adapter 10 of the RCU and the other faultbeing downstream of adapter 15 of the RCU. In this state adapter 10 isconnected to the non-operative ring on the upstream side of the RCU andadapter 16 is connected to the non-operative ring on the downstreamside. Adapter 15 is connected to an isolated operational ring along withany active devices connected to the RCU.

FIGS. 5-17 are flowcharts of the program required in microprocessor 17for implementing functions described above. More particularly, FIGS.5-8, when taken as a whole and combined as indicated, define an overallprocessing routine. FIGS. 9-13 illustrate discrete modules which arecalled as indicated in the drawings, and FIGS. 14 and 15 are routineswhich execute when timers expire.

The terms listed and defined below are used in the flow diagramsillustrated in FIGS. 5-17.

T1A, T1B, T2-T6 and RAT (Recover Activate Timer) identify differenttimers which are set and reset in the flow diagrams.

BNR=Beacon Reconfigure

BNN=Beacon Normal

RR1=Recovery Required 1 (this state exists when PO is receiving BNN ortransmitting BNR)

RR2=Recovery Required 2 (this state exists when PO is receiving andrepeating BNR)

WRAP RI/RO=Switch State FIG. 4D

WRAP RO=Switch State FIG. 4C

WRAP RI=Switch State FIG. 4B

UNWRAP RI/RO=Switch State Change FIG. 4D to 4A

UNWRAP RI=Switch State Change FIG. 4B to 4A or 4D to 4C

UNWRAP RO=Switch State Change FIG. 4C to 4A or 4D to 4B

LM=LAN MANAGER (a program product such as the IBM Network LAN ManagerProgram, which gathers information relative to operation of the networkand issues commands to the devices on the network)

MRO=Merge Ring Out flag (this flag is set to "0" any time the ring in isinoperative)

MRI=Merge Ring In flag (this flag is set to "0" any time the ring in isinoperative)

AMP=Active Monitor Present (A message specified in the IEEE 802.5Standard)

RF=Recovery Flag

RC=Recovery Counter

NRE=Non-Recoverable Error Flag

NAUP=Next Active Upstream Port

AM=Attachment Module (a segment switch and the attached ports)

AMT=Attachment Module Under Test

In the description of FIGS. 5-17 it will be assumed that the network isconfigured as shown in FIG. 1 and includes at least three RCUs. If abreak occurs between RCUs, one RCU will be immediately downstream of thebreak on primary ring P, another will be immediately upstream of thebreak on the primary ring P and all of the other RCUs will beintermediate the two RCUs defined above. If a break occurs between thePI and PO adapters, the RCU detecting the break will assume thereconfiguration switch state illustrated in FIG. 4D and all the otherRCUs will be intermediate to the break.

The program is started at power on in FIG. 5. As indicated in the startblock, the flag bits DIS₋₋ TX, MBF and TBF are set OFF for adapters PI,P0 and S. At this time switch 11 is in the state illustrated in FIG. 4Dand the program branches to a subroutine of the main program at (G) FIG.8B. Assuming there are no failures, the S adapter would have received anAMP frame 801, timer T6 would be reset 802 and the program would branchto 6. Since RI is wrapped 803, MRI=1 804, and PI has received an AMP805, RI is unwrapped 806.

The program branches back to 6. At this time RI is not wrapped. IF PO isreceiving 807 or transmitting 808 a BNN, timer T6 is loaded and started809 if it is not active 809'. If timer T6 is active it goes to 810;however, RO is assumed operational and the program descends to 810directly where S has, as previously set forth, received an AMP frame.The RO is operational and MRO=1, 811. RO is unwrapped and timers T_(1A)and T_(1B) are reset 812. The program branches back to A on FIG. 5 whereTBF (PO and S) are set on 502' and it loops through blocks 502-505 inthe absence of failure in RI, RO or an internal (between PI and PO)failure. At this time the RCU switch state is illustrated in FIG. 4A.

The network can be operating as a dual ring network or as a single ringreconfigured network. It will only operate as a dual ring network whenall of the RCUs are powered on and no faults are present. If a singlefault exists or if a single or multiple adjacent RCU(s) are powered downa single ring which includes part of the secondary ring is operational.If there are more than one fault or two or more non-adjacent RCUs arepowered down two or more independent single rings may be operational.The description below is unaffected by these conditions and will applyto all RCUs which are similarly situated.

If a fault or break takes place on the primary ring, one RCU will beimmediately downstream of the break and another will be immediatelyupstream of the break. Any other RCUs will be located intermediate theupstream and downstream RCUs.

We will first consider the RCU downstream of the break. The PI adapterwill detect the break and transmit a BNN and the condition will becommunicated to the program. The PO adapter will not repeat the BNN butwill transmit a BNR downstream on the primary ring and report this tothe program causing it to enter the state RR1; thus the program FIG. 5will go from block 502 to block 507. In FIG. 5 two recovery requiredconditions RR1 and RR2 are utilized. RR1 exists when the PO adapter iseither transmitting BNR frames or receiving BNN frames and RR2 existswhen the PO adapter is receiving and repeating BNR frames. Thisinformation is provided to the program by the PO adapter. At this time,timer T1A is not active and timers T1A and T1B are loaded and started508 and the program returns to 1. The program will wait for timer T1A toexpire 509 before proceeding. Once timer T1A has expired, the programsets the DIS₋₋ TX bit on and the TBF bit off in the S adapter causingthe S adapter to transmit BNN frames and the PO adapter will transmitBNR frames, since its TBF bit had been previously set on and it hadreceived the BNN frames transmitted by the PI adapter 510. If POreceives its own BNR frame 511, timers T1A and T1B are reset DIS₋₋ TX(s)is set OFF 512 and the program returns to 1. Since PO received its ownBNR, the primary ring is intact and whatever caused the break indicationhas disappeared.

If PO is receiving a BNR from another adapter 513 timer T3 is loaded andstarted, DIS₋₋ TX(s) is turned OFF and TBF(s) is turned ON, 516 and theprogram branches to B in FIG. 6; however, in the instant example thiswill not happen and in block 513' the program determines if the Sadapter has received PO's BNR. If it has the timer T5 is reset, DIS₋₋TX(s) is turned OFF and ring in (RI) and ring out (RO) are wrapped 514by changing the reconfiguration switch 11 from the state illustrated inFIG. 4A to the state illustrated in FIG. 4D 514. The program thenbranches to C on FIG. 7. At this time it executes an internal recoverymodule 701 and TBF (PO and S) will be set OFF 701'. The internalrecovery routine is shown in detail in FIG. 9. The function of thismodule under the assumed conditions is to check the integrity of theisolated ring which includes the S adapter and all of the active portscontrolled by this RCU see FIG. 4D. If, as assumed, no faults exist onthe isolated ring, the S adapter will receive an AMP frame 702, the ringout (RO) is operational and MRO equals 1 (703). Ring out (RO) isunwrapped 704 and the reconfiguration switch changes from the stateillustrated in FIG. 4D to the state illustrated in FIG. 4B, timers T1Aand T1B are reset 705 and the program returns to A FIG. 5.

We will now consider the RCU immediately upstream of the break or faultsince the action it takes causes the return of PO's BNR to S at thedownstream RCU and the subsequent formation of the single operating ringdescribed above. Turning to FIG. 5 the upstream RCU will have gonethrough the same power-on self-test previously described for thedownstream RCU and the description will start a A of FIG. 5.

Under the assumed condition the program exits blocks 502, 503 and 504 onthe negative side. Since the PO adapter is receiving and repeating BNRframes, RR2 is satisfied and block 505 exits on yes (Y). Timer T1B ischecked in block 515 and is not active. Timer T3 is loaded and started,the S adapter flag bits are changed 516 and the program branches to B onFIG. 6.

In FIG. 6 block 601 checks if the S adapter received a BNR. If it hastimers T1A and T1B are reset and the program branches back to A on FIG.5. A yes response in blocks 602, 603 or 604 causes timer T4 to be loadedand started and the MBF flag bit in the S adapter set ON 605. If all arenegative and the S adapter detects a break 606 timer T6 is reset 607 andthe program wraps RO and branches to D on FIG. 8. If any of theconditions specified in blocks 602-604 are satisfied and timer T4 isstarted, block 608 will for the immediate upstream RCU be negative andthe program will wrap RO, reset timer T6, set MBF(s) OFF 609 and branchto D on FIG. 8 if either the timer T4 expires 610 or the S adapterdetected a break 611.

After branching to D, the TBF flag bit in PO and S are set to OFF 803'and under the assumed conditions, the program will exit block 803 on thenegative side and loop through blocks 807, 808, 810 and 813-815 while itis in the Wrap RO (FIG. 4C) state. While in this loop the program willmonitor the input from the S adapter (block 810). When S receives an AMPframe, it indicates that the secondary and primary ring paths betweenthe S adapter of the upstream RCU and the PI adapter of the downstreamRCU is operative, MRO will be 1 and it will exit the loop through block811 and 812 (where RO is unwrapped). It returns to A on FIG. 5 where itwill loop through blocks 502-505 and in the absence of any new faults,the program continues to loop through blocks 502-505.

The downstream RCU, as previously described, returns to A in FIG. 5after unwrapping RO (FIG. 7 block 704). This leaves RI wrapped. Whilelooping through blocks 502-505, it waits for PI to receive an AMP frame504. When this occurs, it indicates that the fault in the primary andsecondary ring between its RI and the upstream RCU's RO has disappearedand if MRI=1 504', RI is unwrapped in block 506. Since the upstream RCUunwraps its RO under the same condition, S received AMP, the dual ringis now restored.

The RCUs intermediate the upstream and downstream RCUs follow the samepath in FIG. 5 as the upstream RCU. In FIG. 6, however, the receipt ofthe BNR by the S adapter will cause the program in that RCU to go backto A of FIG. 5 after resetting timers T1A and T1B in block 612 or 613depending on whether it receives the BNR before timer T3 or T4 expire.These RCUs do not change the switch state but remain in the stateillustrated in FIG. 4A.

When a PO adapter detects a break, 503, the timer T2 is loaded andstarted 517 and the program goes into the loop 510, 511, 513' and 518.If timer T2 expires before PO receives a BNR frame or S receives PO'sBNR frame, RI/RO are wrapped, DIS₋₋ TX(s) is turned OFF 519 and theprogram branches to E on FIG. 8. At this time the internal recoverymodule (FIG. 9) is called 817. This module is used to locate and/orbypass failed components in the isolated ring (FIG. 4D). If thecomponent is bypassed, the S adapter will receive an AMP frame 801 andreset timer T6 802 and go to 6 FIG. 8 as previously described.

If, on the other hand, a successful bypass is not possible, the programwill loop through blocks 818, 819 and 801 until either the PO adapterreceives an AMP frame 818 in which case timer T5 is reset 820 and theprogram branches to C on FIG. 7.

When the program branches to 5 on FIG. 7 from 820 on FIG. 8, RI/RO arewrapped and a non-recoverable error exists on the isolated ring. In thisevent, S will not receive an AMP frame 702 and the program will loopthrough blocks 707-711 and 702 pending correction of the faults or iftimer T5 expires 709, in which case the program branches to E on FIG. 8.If the program exits blocks 707 or 708, yes Timer T5 is checked to seeif it is active 713. If it is not active, it is loaded and started 713and returns to 5. If timer T5 is active, the program descends to block709 and proceeds as described above. Timer T5 will be reset in block 714if adapter PO receives a BNR frame. A branch to E on FIG. 8 will alsotake place if the PO adapter detects a break 710. In block 703 MRO notequal 1 (i.e. RO inoperative) will reset timer T6 and branch to D onFIG. 8.

In FIG. 8 if timer T6 expires 813 or the PO adapter detects a break 814,RI is wrapped 821 and the program branches to block 817 to perform aninternal recovery routine illustrated in FIG. 9 and proceeds from thereas previously described.

The Internal Recovery module illustrated in FIG. 9 is only entered whenRI/RO is wrapped (FIG. 4D). In block 901 the recovery flag is set equalto 1. If the RAT timer is active 902, it is reset in block 903 and NREis set to false in block 904. If the internal ring is recovered 905(i.e. S receives an AMP frame) the recovery counter value is examined inblock 906. If it is not 3, which will be the case if no fault exists onthe isolated ring, the RAT timer is loaded and started 907 and any badpieces are reported 908 to a LAN manager, if resident on the ring 908and the RF flag is set to zero 908'. The recovery counter RC will onlygo to three when the fault or error is not found or located, in whichcase ring error not found is reported 909 and the recover retry counteris started 910. At this time the program returns to the point from whichit branched.

If a fault or error exists on the isolated ring, the program willdescend through blocks 911 and 912 since NRE is false and RC is assumedto be zero at this time. The recover counter, RC, is reset to zero whenthe RAT timer expires, see block 1501 in FIG. 15. This counter is usedto prevent oscillations when a fault is not found while the ring isisolated but reoccurs as soon as reconnection takes place. The counterlimits this to three tries. At this time the program calls Find. Fault913.

Find Fault, FIG. 10 is initiated with an 18 second wait 1001. If theinternal ring is recovered 1002, the program returns to block 914 onFIG. 9. Since no faults were found, RC is incremented by 1 in block 915.If any ring components had been deactivated (bypassed) that informationwould be saved in block 916. Under these circumstances the program wouldreturn through blocks 905, 906, 907 and 908.

If the internal ring has not recovered in block 1002, the more typicalsituation, the Next Active Upstream Port (NAUP) is removed 1003. TheNAUP is the first active port upstream of the beacon transmitter andgenerally is the port to which the NAUN is connected. The identity ofthe NAUN is supplied by the beaconing device and the program correlatesthat identity with the port as described in detail below. If theinternal ring recovers 1004, the program calls Test Port (1005) FIG. 12.

Test Port (FIG. 12) restores the port, the NAUP's port, in this case1201. If the internal ring fails 1202, the port is removed 1203, flaggedas BAD 1204 and the program returns to 914 where it exits yes andcontinues as previously described.

If in FIG. 10 the ring does not recover (1004) the port to which thebeacon transmitter is connected is removed 1006. If the internal ringrecovers 1007, Test Port 1005 is called. If it fails to recover, all ofthe attachment modules are removed 1008. If the internal ring fails torecover 1009, NRE is set to True 1010 and the program returns to 914 andexits 914 on the yes leg, since a fault has been identified (i.e. the Sadapter or above).

When the internal ring is recovered in 1009, a test frame is transmitted1011 and if the frame test fails 1012, NRE is set to True 1013 whereuponthe program returns to 914 and proceeds from there as previouslydescribed.

If the test frame does not fail, AMT is set to the NAUPs AM 1014 and theAMT is restored 1015. A frame test of the AMT is performed 1016. If theframe test fails 1017 the AMT is flagged as bad and all AMs but the AMTare restored 1018 and the RC flag is set to zero 1018'. The programreturns to 914 and proceeds as previously described. When the frame test1017 does not fail, the AMT is removed 1019, the AMT is incremented by 1modulo 4 (or n where n is equal to the number of AMs) 1020 and the newAMT is examined 1021 to determine if it is the NAUP's AMT. If it is not,the program loops through 1015, 1016, etc.

When the AMT 1021 is the NAUP's AMT this indicates that all AMs havebeen through the loop without a frame test failure being detected at1017 and the program branches to F in FIG. 11 where additional testingtakes place.

In FIG. 11 all of the AMs are restored 1101, AMT is set to equal theNAUP's AM 1102 and the AMT is removed 1103. If the internal ring doesnot recover 1104, the AMT is incremented by 1 modulo 4 1105 and the AMTis examined to see if it is the NAUP's AMT 1106. If it is not, theprogram loops through 1103-1106. If the internal ring does not recoverafter all the AMs have been removed, NRE is set to true 1107, the errorcondition is posted 1108 and the program returns to 914.

If the internal ring recovers 1104 the program calls Test Module (FIG.13) 1109. In Test Module (FIG. 13) all of the ports on the AMT areremoved 1301, the AMT is restored 1302 and port 20 (or n where n is thehighest numbered port) is restored 1303. If this port is active 1304,the Test Port Module (FIG. 12) previously described is called andexecuted 1305. If the port is flagged as bad 1306 the program returns to1110 on FIG. 11 where all of the good ports and modules are restored andthen returns to 914. If on the other hand the port was neither active1304 or bad, the port number is decremented by 1 1307 and if the portnumber is not 0 1308 the program loops back to 1304. When the portnumber equals 0 the program returns to 1110 on FIG. 11.

The Recovery Retry Timer Expired Module FIG. 14 regulates the number oftimes and the frequency of the attempted recoveries. As previouslydescribed, the timer is started in block 910 of FIG. 9 when the ringerror is not found. Thus another attempted recovery must await theexpiration of the timer. Once the timer has expired, RC is set to 2 1401(FIG. 14) and RF is set to 0 in 1402. If the internal ring fails 1403,internal recovery 1404 (FIG. 9) executes as previously described. If itdoes not fail, Ring Error Not Found is cleared 1405 and RC is set to 0in 1406.

The flow diagrams illustrated in FIGS. 16 and 17 define the programmodules which correlate the ports to the specific terminals or devicesconnected to the lobes or wires connected to the ports whereby theprograms can create tables which identify the specific port or lobe towhich each terminal or device is connected without any additionalequipment or specific reporting by the connected devices.

The program maintains the four tables listed below which are relevant tothe modules illustrated in FIGS. 16 and 17:

1. Lobe or Port Table

2. Station Build Table

3. Station Good Table

4. Attachment Module or Segment Table

The Lobe or Port Table includes the status (inserted/deinserted) of eachport and is maintained by monitoring the d.c. or phantom currentprovided by the stations or terminals wanting access to the network. Inthe absence of any unique requirements, the program will enable all ofthe ports and the presence of the d.c. current from a station willinsert the port in the series circuit. In addition, the program via bus18 is informed that the port is active and marks the Port Table entrycorresponding to that port as inserted. In a like manner, if the d.c.current is removed, the port is bypassed and the corresponding entry forthat port in the table is marked deinserted. In addition, if aspreviously described, a port is bypassed or inserted the table isupdated to indicate the status of the port. The Attachment Module (AM)or Segment Table is similar and includes the status(inserted/deinserted) for each of the four Segments or AttachmentModules which are under control of the processor as previouslydescribed.

In addition, the program maintains a Station Build and a Station GoodTable. Each of these tables includes the address or identity of thestation connected to each lobe or port. When the program firstrecognizes a station connected to a port, the station identity isinserted into Build Table and upon a subsequent recognition, the stationidentity is entered into the Good Table. The information in the GoodTable is considered reliable and is used for network management.

FIG. 16 illustrates an interrupt routine for maintaining the insertstatus of the AMs and their ports and is executed every 64 MS 1601. Whenthe timer expires, the current AM is set equal to 1 1602 and if thecurrent AM is less than or equal to 4 (since there are 4 AMs) 1603 theprogram gets the current AM insert status from the AM Table 1604. If thecurrent AM has just deinserted 1605, the program clears the Lobe andBuild Tables and changes the insert status in the Good Table for all ofthe ports connected to the AM 1606.

If the current AM has not just deinserted, it checks the AM Table todetermine if it is inserted 1607. If it is not inserted, the current AMvalue is incremented by 1 1608 and the program loops back to 2 where theabove steps are repeated for the next AM. When the current AM isinserted, the current lobe or port is set to 1 1609 and the programenters a loop to maintain the insert status of the twenty ports or lobesconnected to the current AM.

In block 1610 the program checks to see if the current lobe is less thanor equal to twenty (20). If it is not, this indicates that all ports inthe current AM have been maintained and the current AM is incremented by1 in block 1611 and the program returns to 2 to process the next AMprovided the next AM is equal to or less than 4 in block 1603. When thecurrent lobe is twenty or less, the program gets the current lobe insertstatus from the Lobe/Port Table 1612 and checks to see if it has beenjust deinserted 1613. In order to make this determination, the tableincludes the past history of the last three status changes or states(eg. 110 or 100 where 1=inserted and 0=deinserted). If the lobe has justdeinserted the program clears the Lobe and Build Tables and changesinsert status for the port in the Good Table for the current lobe 1614and increments the Current Lobe by 1 1615 and then returns to 3 toprocess the tables for the next lobe.

If the lobe has not just deinserted, the lobe Table is checked 1616 tosee if it is inserted. If it is the Lobe Table status for the currentlobe is updated 1617 and the current lobe is incremented by 1 1615.

FIG. 17 is a flow chart of the program module which handles ring pollframes or messages. As previously described, the ring poll frameincludes the address or identity of the source and the address oridentity of the next active upstream neighbor station (NAUN), however,it does not include any information as to which ports or lobes these twostations are connected to. This module generates the build and goodtable which correlates the sequential addresses or identities with theinserted or active ports or lobes.

Each of the adapters 10, 15 and 16 provides the processor 17 with a copyof ring poll frames which it generates and adapter 16, in addition,provides a copy of all ring poll frames regardless of source to theprocessor 17. The program module starts at 1 and examines copies of ringpoll frames received over the bus 19. If the source of the frame is thering in adapter 10/16 (depending on the current configuration of theswitch 11, see FIGS. 4A-C) the program will exit block 1701 on the yesleg; otherwise it will loop. In the description which follows, theadapters will be referred to as ring in or ring out media access control(MAC) in accordance with prevalent practice in this technology. When aring poll frame from the ring in MAC is received, the program sets avariable O.S.A. (Old Source Address) equal to the source address of thering in MAC 1702. The program waits for a received frame in 1703 andwhen one is received, the NAUN addresses are compared 1704. When theycompare the frame is a properly sequenced ring poll frame and the sourceaddress is compared with the ring out MAC address 1705.

When the source address does not equal the ring out MAC address 1705,the O.S.A. is set equal to the source address of the frame and a searchis made for the next inserted port 1706. If one is not found 1707 theBuild Table is cleared 1708 and the program returns to 1, since this isa clear error condition and any new information in the current ring pollcycle is not reliable. If one is found, the Good Table is checked 1709for the source address and if there no further action is required andthe program goes back to 2.

When the source/station address is not found in the Good Table, theprogram checks the Build Table 1710 and, if the source address is in theBuild Table, the address is copied into the Good Table 1711 and theprogram goes back to 2 to process the next received frame. When thestation address is not found in the Build Table 1710, it is copied intothe Build Table 1712 and the program returns to 2.

If the source address equals the ring out MAC address 1705, the programsearches for the next physically inserted port 1713. If one is found1714, an error condition is indicated and the Build Table is cleared1715 before the program returns to 1 where it waits for another ringpoll cycle to start. When no additional inserted ports are found 1714,this indicates a successful completion of the current ring poll cycleand the program returns to 1 to await another. Blocks 1707 and 1714detect, respectively, under and over mismatches between the number ofinserted ports and the number of ring poll frames received between thering in MAC and the ring out MAC addresses.

FIGS. 18-20 are flow diagrams of the addition code required in adapters10, 15 and 16 (PI, PO and S, respectively) to respond to the additionalflag bits added to those currently used in the IBM Token Ring Adapter.The flag bits are transmitted by the processor 17 over the bus 19 andare set "on" and "off" as previously described in FIGS. 5-8.

When DIS₋₋ TX is set on FIG. 18, 1801 the adapter transmits (XMT) aBeacon Normal (BNN) frame 1802. If DIS₋₋ TX is turned off 1803, theadapter discontinues transmitting (XMTG) Bnn frames 1804 and returns toStart. If DIS₋₋ TX is not turned off, it continues transmission of BNNframes.

In FIG. 19 if the adapter is transmitting a BNN 1901 and the TBF flag ison 1902, it will transmit a BNR frame 1903. If the TBF flag is not off1904 and the beacon condition still exists 1905, it will loop andtransmit BNR. If TBF is off 1904, the adapter will discontinuetransmitting BNR 1906 and transmit the standard IEEE 802.5 claim tokenframe 1906 and return to start.

In FIG. 20 if the flag bit MBF is on 2001, the adapter will transmit aBNR frame 2002 and continue to do so until as long as MBF is on;however, when MBF is turned off 2003, the adapter will stop transmittingBNR frames 2004 and return to start.

The foregoing and other features and advantages of the invention will beapparent to those skilled in the communication art.

We claim:
 1. A reconfiguration unit for attaching to a dual ring networkwhich includes a primary ring (PR) carrying signals in a first directionand a secondary ring (SR) carrying signals in a second oppositedirection and for selectively interconnecting said PR and SR when signaltransmission in the dual ring network is impaired comprising:a pluralityof station access ports for attaching terminal devices; a first andsecond input for receiving signals from said PR and SR respectively; afirst and second output for transmitting signals on said PR and SRrespectively; first, second and third adapter means for selectivelyreceiving signals from the network, retransmitting signals andgenerating signals; first switching means operatively coupled to thestation access ports, said first switching means selectively connectingat least one station access port to said network; a programmed processorconnected to said first, second and third adapter means for receivingthe signals therefrom and generating control signals related thereto;and a second switching means connected to said first and second inputsand outputs, said first, second and third adapters and said firstswitching means; said second switching means receiving the controlsignals and interconnecting selected ones of said first and secondinputs and outputs, said first, second and third adapters and said firstswitching means so that the dual ring network is reconfigured into oneof a plurality of reconfigurable states.
 2. A reconfiguration unit asset forth in claim 1 in which said second switching means in response toa first control signal from said programmed processor connects saidfirst adapter means, said at least one port and said second adapter inseries between the first input and the first output; andconnects thesecond input, said third adapter and the second output in series.
 3. Areconfiguration unit as set forth in claim 1 in which said secondswitching means in response to a second control signal from saidprogrammed processor connects said third adapter means, said at leastone port and said second adapter means in series between the secondinput and the first output; andconnects the first input, said firstadapter and the second output in series.
 4. A reconfiguration unit asset forth in claim 1 in which said second switching means in response toa third control signal from said programmed processor connects saidfirst adapter means, said at least one port and said second adaptermeans in series between the first input and the second output;and,connects the second input, said third adapter means and the firstoutput in series.
 5. A reconfiguration unit as set forth in claim 1 inwhich said second switching means in response to a fourth control signalfrom said programmed processor connects the first input, said firstadapter means and the second output in series;the second input, saidsecond adapter means and the first output in series; and forms anisolated ring by connecting said third adapter means in series with saidat least one port.
 6. A reconfiguration unit as set forth in claim 1 inwhich said second switching means in response to a fifth control signalfrom said programmed processor connects the first input, said firstadapter means and the second output in series;the second input, saidthird adapter means and the first output in series; and, forms anisolated ring by connecting said second adapter means in series withsaid at least one port.